(1) Field of the Invention
This invention relates to clear liquid skin cosmetic compositions which contain a solubilizing agent comprising an 18.alpha.-glycyrrhizin (18.alpha.-glycyrrhizic acid sodium salt, potassium salt or ammonium salt) that is a novel compound obtained by isomerization of a conventional glycyrrhizin (i.e., any form of 18.beta.-glycyrrhizin extracted from licorice root) or a glycyrrhizin composition consisting of an 18.alpha.-glycyrrhizin and an 18.beta.-glycyrrhizin. More particularly, it relates to such clear liquid skin cosmetic compositions in which the difficulties inherent in the use of a solubilizing agent comprising a conventional glycyrrhizin (18.beta.-glycyrrhizin) alone or a synthetic surface-active agent are overcome.
(2) Description of the Prior Art
It is well known that aqueous solutions of conventional glycyrrhizins (18.beta.-glycyrrhizic acid sodium salt, potassium salt and ammonium salt) present peculiar phenomena. That is, they form a gel and show a reduction in solubility in the pH range of from 2.5 to 6.
A skin cosmetic of the gel type has been developed by utilizing the above-described gelation phenomenon of conventional glycyrrhizins (Japanese Patent Publication No. 21397/1970). However, this skin cosmetic involves a number of difficulties: (1) The gel is lacking in fluidity and scarcely adherent to the fingers, so that it is hard to take out of the container (e.g., the bottle). (2) When applied to the skin, the gel can hardly be destroyed (or liquefied) by pressing it with fingers or palms and, moreover, can hardly be spread evenly because it tends to slip on the skin. (3) The gel undergoes syneresis (i.e., the separation of water) during longterm storage, which impairs its stability and appearance to a marked degree. (4) The bubbles which are formed during the blending step are hard to remove and hence remain in the gel. These difficulties hinder the aforesaid skin cosmetic from being put to practical use.
In addition, Japanese Patent Laid-Open No. 151736/1977 discloses a hormone-blended lotion which is prepared by solubilizing a hormone (such as diethylstilbestrol, estradiol or the like) in an aqueous solution of ethyl alcohol with the aid of a conventional glycyrrhizin (i.e., 18.beta.-glycyrrhizic acid dipotassium salt) and adjusting the pH of the resulting clear solution to 6.1 or greater. However, since the capacity of the conventional glycyrrhizin for solubilizing oily substances (such as hormones and the like) is rather limited, the product tends to become turbid after it has been stored under severe temperature conditions for a long period of time. Moreover, the time required to defoam the product is relatively long because the foam which is formed in an aqueous solution of the conventional glycyrrhizin disappears slowly.
Recently, the so-called acidic skin cosmetics which are adjusted to pH values approximating the pH(4.0-6.0) of the skin and considered to exert a mild action on the skin have come into common use.
However, conventional glycyrrhizins induce a marked degree of gelation and a reduction in solubility in the pH range of from 2.5 to 6 and, in particular, from 3.0 to 5.5. Accordingly, they are utterly useless in the preparation of acidic liquid skin cosmetics which have excellent properties (such as clarity, fluidity, feeling, solubilization stability and the like) and are easy to use. Moreover, the aforesaid gelation and the ensuing sharp rise in viscosity make it difficult to defoam the product. For these reasons, conventional glycyrrhizins have been applicable only to certain types of lotions, for example, the hormone-blended lotion (with a pH value of 6.1 or greater) disclosed in Japanese Patent Laid-Open No. 151376/1977.
In order to overcome the above-described difficulties, the present inventors have performed intensive and extensive studies and have discovered that, if an 18.alpha.-glycyrrhizin (novel compound) of the structural formula (1) which will be given later or a glycyrrhizin composition consisting of such an 18.alpha.-glycyrrhizin and an 18.beta.-glycyrrhizin (conventional glycyrrhizin) of the structural formula (2) which will be given later is used as a solubilizing agent for oily substances, clear liquid skin cosmetic compositions having excellent properties under any desired pH conditions can be readily and advantageously prepared on an industrial scale. The present invention is based on this discovery.